The present invention generally relates to techniques for controlling the automatic transmission used in automobiles, and particularly to a technique for electronically controlling the working oil pressure on one or more clutches that is caused to act at the time of gear shifting.
The automatic transmission generally includes a torque converter coupled with the output shaft of engine, gear units that are connected to the output shaft of the torque converter and have a plurality of driving ranges, and at least two clutches that are acted when the automatic transmission makes gear shifting. Most of the clutches for use in automatic transmission are generally multiple-disk clutches. The gear shifting is performed during the interval in which one clutch is disengaged and the other one is engaged. The clutch is generally driven by oil pressure. Since the transfer torque is changed by the change of the working oil pressure on the engaged clutch, it is important to control the oil pressure of clutch.
A conventional example is given in U.S. Pat. No. 4,922,424 which describes an automatic transmission control for disengaging and engaging clutches by electronically controlling the working oil pressure of clutch. According to this US patent, control values for use in controlling the working oil pressure of the engaged clutch at the time of upshifting and the working oil pressure of the disengaged clutch at the time of downshifting are calculated from a predetermined functional equation based on engine speed, turbine speed and oil pump capacity coefficient of torque converter. The engine speed is the input shaft speed of torque converter, the turbine speed is the output shaft speed of torque converter, and the pump capacity coefficient is determined by the characteristics of the torque converter. The control technique in this US patent enables the working oil pressure of clutch to be correctly and precisely controlled and the transfer torque of clutch to be controlled most suitably to the driving condition of car even though the automatic transmission is changed in its performance after a long time usage.
FIG. 12A is a signal diagram to which reference is made in explaining the controlled operation of the oil pressure type clutch that makes automatic shifting in the conventional automatic transmission. It is assumed that after the car driver depresses the accelerator pedal and makes the car reach a desired cruising speed, he keeps the accelerator pedal as it stands as shown in FIG. 12A by the solid line (a). Under this condition, a shift control command (at (b) in FIG. 12A) is generated according to a shift control program that is determined by the car's speed, and the accelerator pedal angle or throttle opening. The automatic transmission has a shift control program as, for example, shown in FIG. 12B. The curve shown by 1.fwdarw.2 shows the position in which a shift control command to change from the first to second gear is produced. Similarly, the upshifting command curves are shown for the shifting from the second to third, and third to fourth gear. The downshifting command curves are not shown.
When a shift control command (at (b) of FIG. 12A) occurs at time t1 ordering to shift from the second to third gear, the disengaged side oil pressure (not shown) that is applied from the oil pump to the disengaged clutch is reduced. At the same time, the engaged side oil pressure (at (f) of FIG. 12A) to be applied to the engaged clutch is increased. This engaged side oil pressure (f) is generated by a hydraulic control system according to an engaged side oil pressure command value ((e) of FIG. 12A). When the engaged side oil pressure (f) is increased enough at time t2, the shifting operation is started. The interval from time t2 to time t5 is the target shifting time, within which the shifting operation is required to complete.
According to the conventional example mentioned above, as indicated by the solid line curves in FIG. 12A when the accelerator pedal is kept at a constant angle during the shifting operation, the shifting operation from the second to third gear is completed at the target time t5, and the turbine speed ((c) of FIG. 12A) is reduced to a low value. However, if the accelerator pedal is further depressed to accelerate at time t3 in the process of the shifting operation, the shifting interval exceeds the target time to extend to time t6 at which the shifting operation ends, as indicated by the broken line. This extension will cause the clutch to wear away or break down, and reduce the performance of the automatic transmission.